Bearing Fit Data

Research into bearing fits to determine if motor shafts are “in tolerance” leads to the conclusion that radial ball bearings with a bore over 100 mm should be classed as “heavy radial load”. This will require the m5 tolerance class as found in ANSI/ABMA Std. 7-1995 Table 1. The m5 and m6 tolerance classifications are the same on the minimum size but the m6 class permits a wider range so we use m6 for radial ball bearings from 170mm to 200mm. This is shown pictorially on page 2 of the ANSI/ABMA Std. 7-1995.

One of the major bearing distributors to our industry recommended fits up to 100mm bore are the same as ours except for bearings with 20mm through 40mm where they permit .0002“ loose fit at the minimum instead of .0001“ tight at the minimum shaft diameter. No knowledgeable electric motor repairman would install a loose fit when an interference fit is required.

Our Cylindrical Roller Bearing Fit Tolerance chart uses “Heavy” radial load since roller bearings are used primarily for heavy radial loads. From experience, a roller bearing was changed in a 250 HP 2880 RPM vertical mount DC motor for a General Electric Company plant. The bearing was a NU 316, 80mm bore. The shaft fit was sized to 3.1505” maximum and 3.1500” minimum from an existing chart. The customer rejected the motor. The shaft fit was changed to be in the range of 3.1511” and 3.1504” with 3.1508” being the ideal size and the shaft was built up with selective brush plating. You can see this is an interference fit of 0.0012”. The motor then passed a vibration test by the customer.

Our Cylindrical Roller Bearing Fit Tolerance chart for 10mm through 40mm uses m5, 45mm through 65mm uses m6, 70mm through 140mm uses n6 and 150mm through 200mm uses p6. This keeps us in strict alignment with recommended tolerances in ANSI/ABMA Std. 7-1995 and will align us with the data published by manufacturers.

Cylindrical roller bearings with a C3 internal clearance have a larger clearance than the same size C3 ball bearing. The increased interference fit will not adversely effect the roller bearing.

If you compare our figures with manufacturer figures you will frequently find a tenth of a thousandth variance. Most manufacturers calculate the maximum and minimums fit in millimeters and micrometers and then convert this data to inch dimensions. This leads to variations from rounding the numbers.

The following two tables show the recommended bearing fit charts for Radial Ball Bearing Fit Data and Cylindrical Roller Bearing Fit Data.

Table #1
Radial Ball Bearing Fit Tolerances

Table #2
Cylindrical Roller Bearing Fit Tolerances

These charts use H6 tolerance class for all housing fits for shaft rotating, outer ring stationary. You should consider Housing Fit tolerance J6 in case of significant vibration and P6 for “shaker screen” motors.
Also note the following:
1. (ABEC 7 Bearing Shaft Fit:
As a general rule, it is recommended that the shaft size and tolerance for seating ABEC 7 bearings be the same as the bearing bore thus producing an average line to line fit.)
2. (Hollow Shaft Bearing Shaft Fit
It is generally necessary to use a heavier interference fit than that used for a solid shaft in order to achieve the same surface pressure between the shaft and the bearing inner race.)